A cargo ship whose mysterious wanderings suggest illicit arms smuggling near U.S. shores shows North Korea doesn't need missiles to threaten America — which must strengthen its national defense and never let its guard down, especially in its own backyard.

Writing for Forbes, Claudia Rosett of the Foundation for Defense of Democracies brings to light the North Korean-flagged freighter Mu Du Bong. In late June, it “called at Cuba, then vanished from the commercial shipping grid for more than a week,” likely by turning off a required transponder. Reappearing at Havana on July 10, it was last known to be “cruising the Gulf of Mexico, not all that far from ... the coast of Texas.”

Ms. Rosett cites “disturbing similarities” to “another North Korean freighter, the Chong Chon Gang.” It was caught last summer violating United Nations sanctions on North Korea by trying to smuggle through the Panama Canal “an illicit load of weapons” it had picked up in Cuba. It carried about “240 tons of arms and related materiel, including two disassembled MiG-21 jet fighters, additional MiG engines, surface-to-air missile system components, night vision goggles and ammunition” — all concealed by “more than 200,000 bags of Cuban sugar.”

Rosett says the United States could impose its own sanctions on North Korean vessels in general. But the real lesson here is the need for greater U.S. vigilance against such threats close to its shores — especially so long as the Castro regime welcomes North Korean port calls.